Lafarge Africa said the plant "was the target of an intrusion by people who were strangers to the plant", but it added that "the situation has now stabilized and there is no report of any injury to employees or damage to the plant."

Tanimu Ayuba, a witness who lives near the plant, said the fighters came in six pick-up trucks and were firing weapons at the site, near Gombe state. A security source confirmed the account.

"The insurgents had a field day and unhindered access to the quarry site of the company. They took as much dynamite as they could and left the place," Ayuba said.

Another witness, Abdullahi Maina, said he saw the insurgents leaving with dynamite loaded on their trucks, adding that they did not kill anybody in the attack.

Boko Haram's bloody five-year battle for an Islamic state has killed thousands and ruined the economies of the worst affected areas, but the broader economic impact has been limited by the fact that its core areas of operations are in economically marginalized places.

But the attack on the Lafarge facility underscores the security risk posed by the group to Africa's biggest economy and leading energy producer. The Islamists have set off several bombs in cities across the country this year.

Boko Haram militants have seized control of the northeast Nigerian town of Mubi, killing dozens of people and forcing thousands to flee

Nigeria's government announced last month that a ceasefire had been agreed with Boko Haram and that talks were under way in neighboring Chad for the release of more than 200 Nigerian schoolgirls abducted in April by the Islamist rebels.

But recent attacks across Nigeria's northeast by suspected Boko Haram fighters have raised serious doubts about a deal.

Gombe state police spokesman Fwaje Atajiri confirmed the attack and said four policemen were wounded during it. He had no further details.